In the process of designing a circuit, a circuit designer may insert buffers between signal sources and sinks (where signal sources are components that originate signals and sinks are components that receive the signals). Such buffers serve to accelerate the propagation of the signals so that the signals arrive at the sinks faster, and they also serve to reduce signal transition time, which in turn, improves slew rate. Through the addition of buffers, a circuit designer makes it possible for the circuit to meet various timing requirements.
After buffer insertion, a timing analysis is often performed on the resulting circuit. After such timing analysis is performed, it is often discovered that certain hold time constraints are violated. Put another way, it is often discovered that the signals have been accelerated too much so that they arrive at the sinks too fast; as a result, the hold time requirements of certain components (e.g. flip flops) are violated. In such a case, the circuit has to be revised to rectify the hold time violations.
One possible approach to revising the circuit is to insert one or more delay elements close to each of the sinks at which there is a hold time violation. These delay elements serve to delay signal propagation, which in turn, prevents the signal from arriving at the sink too early. While this approach may solve the timing violation problem, it often requires significant work and resources, including determining where the delay elements may be legally placed (without overlapping with existing blocks and gates), re-extraction of RC parasitics, and another round of timing analysis. This can add significant uncertainty to the finalization of the circuit design. Another approach that can be taken is for the circuit designer to re-do the buffer insertion process (e.g. re-analyze the circuit and put different buffers in different places in the circuit). Again, while this may eventually solve the timing violation problem, it requires significant effort and considerable amounts of resources, which add cost and delay to the design process. Besides, hold time analysis and timing violation removal are often performed near the end of a design cycle; thus, only incremental changes are usually allowed at this stage. Hence, re-doing the buffer insertion process is not a preferable approach to fixing hold time violations.
A more desired approach to solving the timing violation problem is to use the current circuit design with the current buffer insertions, and to swap or replace certain buffers with other buffers having different timing characteristics. For example, if an additional delay of 5 picoseconds (ps) is required, then a repeater (which is a type of buffer) having a delay of 15 ps may be replaced with a repeater having a 20 ps delay. Component replacement is often simpler from a design standpoint because it usually does not involve any placement or physical design considerations. Because of this, component replacement is often the approach of choice.
Currently, component replacement is carried out manually by the circuit designer. That is, the circuit designer analyzes the circuit design, makes an educated guess as to which components should be replaced with which other components to rectify the timing violations, and then performs the component replacements in the circuit design. This ad hoc approach to component replacement is problematic because it is labor intensive, error prone, and not likely to produce an optimal solution in which the least number of components are replaced. Because of these shortcomings, an improved approach is needed for implementing component replacement.